Engines are usually designed with the ability to deliver a peak output, although most engine operation is performed well below this peak value. As such, it can be beneficial to operate with some cylinders inducting air without fuel injection, and other combusting air and injected fuel, as described in U.S. Pat. No. 6,568,177. In one example, the system uses lean combustion, and a NOx trap to avoid NOx emissions.
However, the inventors here have recognized that, in some cases, it is desirable to obtain fuel economy improvement without requiring a NOx trap. However, in these cases, there is the disadvantage that NOx emissions can be generated if exhaust gasses from cylinders combusting is mixed with cylinders pumping air, where the mixed gasses are treated in a three way catalyst. As such, different exhaust paths are provided for different cylinder groups.
The inventors herein have recognized, however, that in the case of a V-8 engine, disabling a bank of cylinders that have a common exhaust pathway can result in increase noise and engine vibration, since a V-8 may not be balanced in a bank cut-out condition.